This invention relates in general to defect detection, and, in particular, to an improved system for detecting anomalies on surfaces, such as particles and surface-originated defects such as crystal-originated particles (xe2x80x9cCOPsxe2x80x9d), surface roughness and micro-scratches.
The SP1TBI(trademark) detection system available from KLA-Tencor Corporation of San Jose, Calif., the Assignee of the present application, is particularly useful for detecting defects on unpatterned semiconductor wafers. While the SP1TBI system provides unsurpassed defect sensitivity on bare wafers or unpatterned wafers, this is not the case when it is used for inspecting wafers with patterns thereon such as wafers with memory arrays. In this system, all of the radiation collected by a lens or ellipsoidal mirror is directed to a detector to provide a single output. Thus, since pattern on the wafer will generate Fourier and/or other strong scattering signals, when these signals are collected and sent to the detector, the single detector output becomes saturated and unable to provide information useful for detecting defects on the wafer.
Conventional techniques for detecting defects on wafers are either tailored for the inspection of patterned wafers, or for inspecting unpatterned or bare wafers, but not both. While inspection systems for detecting patterned wafers may be also used for inspecting unpatterned wafers, such systems are typically not optimized for such purposes. Systems designed for the inspection of unpatterned or bare wafers, on the other hand, may have difficulties handling the diffraction or other scattering caused by the patterned structures on patterned wafers, for reasons such as those explained above.
For the inspection of patterned wafers, entirely different inspection systems have been employed. One commercial system, known as AIT(trademark) inspection system, is available from the Assignee of the present application, KLA-Tencor Corporation of San Jose, Calif.; such system is also described in a number of patents, including U.S. Pat. No. 5,864,394. In the AIT system, spatial filters are employed to shield the detectors from the diffraction or scattering from the patterned structures on the wafer. The design of such spatial filters can be based on prior knowledge of the patterned structures and can be quite complex. Furthermore, this system utilized a die to die comparison process in order better to identify the presence of a defect.
None of the above-described instruments is entirely satisfactory for the inspection of patterned wafers. It is therefore desirable to provide an improved defect detection system for patterned wafers in which the above difficulties are alleviated. To further economize on the space required for inline inspection, it is desirable to provide an instrument that can be optimized for both unpatterned and patterned wafer inspection.
Chemical mechanical planarization (CMP) has gained wide acceptance in the semiconductor industry. The CMP process, however, also creates many types of defects that can significantly impact the yield of an integrated circuit (IC) device if the defects are not properly controlled. Among the CMP defects, the micro-scratch has a strong impact on IC yield. Therefore, it is desirable to be able to detect and differentiate micro-scratches and other CMP defects from particles.
One important parameter for monitoring the quality of unpatterned or bare films on silicon wafers is the surface roughness. Surface roughness is typically measured by an instrument such as the HRP(copyright) instruments from KLA-Tencor Corporation, the Assignee of the present application, or by means of other instruments such as atomic force microscopes or other types of scanning probe microscopes such as scanning tunneling microscopes. One disadvantage of such instruments is the slow speed of their operation. It is therefore desirable to provide an alternative system which may be used for giving a measure of surface roughness at a speed much faster than the above-described instruments.
One aspect of the invention is based on the observation that the collectors in the SP1TBI instruments preserve the azimuthal information of the scattered radiation by the surface inspected. Thus, by segmenting and directing the scattered radiation collected by the type of collectors used in the SP1TBI instruments at different azimuthal positions to separate collection channels, the above-described difficulties are overcome so that an instrument may be constructed which is also optimized for the detection of patterned wafers. In this manner, a compact instrument can be achieved for measuring defects of patterned wafers. In addition to the ellipsoidal mirror used in the SP1TBI instruments, other azimuthally symmetric collectors may be used, such as a paraboloidal mirror used together with one or more lenses.
As in the SP1TBI system, the surface inspection system of one aspect of this invention collects radiation scattered from the surface by means of a collector that collects scattered radiation substantially symmetrically about a line normal to the surface. By directing to different channels the collected radiation scattered at different azimuthal angles about the line or another direction, these channels will carry information related to scattered radiation at corresponding relative azimuthal positions of the scattered radiation. Preferably, the channels are separated from each other by separators to reduce cross-talk. The collected scattered radiation carried by at least some of the channels may then be used for determining the presence and/or characteristics of anomalies in or on the surface. In addition, the multiple views of the same event can significantly facilitate the process of real time defect classification (RTDC).
In the above-described scheme, if only a portion of the collected radiation is directed to the different channels, while another portion of the collected radiation at different azimuthal angles are directed to a single detector for providing a single output as in the conventional SP1TBI scheme, the system can then be used for inspecting both unpatterned and patterned wafers. In other words, if the SP1TBI scheme is modified by diverting a portion of the collected radiation in the manner described above to different channels while preserving azimuthal information, a versatile tool results that can be optimized for the inspection of both unpatterned and patterned wafers. In this manner, semiconductor manufacturers no longer have to employ two different tools, each optimized for the detection of patterned or unpatterned wafers.
In the above-described scheme, since collected radiation at different azimuthal angles about the line normal to the surface are directed to different collection channels and converted into separate signals, the signals containing pattern diffraction can be discarded and the remaining signals not containing pattern scatter may then be used for the detection and classification of anomalies in or on the surface of the wafer. While the above-described systems are particularly useful for the inspection of semiconductor wafers, they can also be used for he inspection of anomalies on other surfaces such as flat panel displays, magnetic heads, magnetic and optical storage media and other applications.
Another aspect of the invention is based on the observation that the radiation collected by a collector (such as the one described above) may be filtered by means of a spatial filter having an angular gap of an angle related to the angular separation of expected radiation components scattered by pattern on the surface. In this manner, the filtered radiation at some relative positions of the surface relative to the filter will contain information concerning defects of surfaces unmasked by pattern scattering that would interfere with the measurements. When such radiation is detected by the detectors, the detector outputs can then be used for detecting the presence and/or characteristics of anomalies in or on the surface.
The SP1TBI tool or the above-described systems may be used for distinguishing between particles and micro-scratches caused by CMP. Scattered radiation along directions close to the normal direction is collected by a first detector and radiation scattered along directions away from the normal direction is collected by a second detector. A ratio is then derived from the outputs of the two detectors to determine whether an anomaly on the surface is a micro-scratch or a particle.
The CMP micro-scratches tend to scatter radiation from an oblique incident beam in the forward direction while particles tend to scatter such radiation more evenly. Radiation scattered by the surface along forward scattering directions is collected separately from scattered radiation in other scattering directions. Two different signals are derived from the separately collected scattered radiation and compared for determining whether an anomaly on the surface is a micro-scratch or particle.
In another aspect of the invention, an S-polarized radiation beam and a P-polarized radiation beam are provided sequentially in oblique direction(s) to the surface during two different scans of the surface. The radiation scattered by a defect during the first and second scans is collected to provide a pair of signals indicative of the scattered radiation of two different incident polarizations. The pair of signals is then compared to a reference to determine whether an anomaly on the surface is a micro-scratch or particle.
In order to speed up the process for determining the surface roughness of thin films, a database correlating haze values with surface roughness of thin films is provided. The haze value of the surface is then measured by a tool such as the SP1TBI or one of the above-described systems, and a roughness value of the surface may then be determined from the measured haze value and the database. For example, the database may be compiled by means of a tool such as the SP1TBI or one of the above-described systems for measuring the haze values of representative thin films and another tool such as an HRP(copyright) profiler or other type of profilometer or a scanning probe microscope for measuring the surface roughness of such films.
Any one of the above-described aspects of the invention may be used individually or in any combination to achieve the advantages described herein.